Aluminum anode alloy for primary high power density alkaline fuel cells and batteries

ABSTRACT

A tin and mercury free aluminum alloy of specific composition for use as annode in fuel cells and electrochemical couples and particularly in a silver oxide-aluminum battery comprising high purity aluminum alloyed with gallium and magnesium in specified quantities. The aluminum in combination with the gallium provide desired unique electrochemical properties. The magnesium, when the anode material is suitably heat treated, increases the tensile modulus of the alloy.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout they payment of any royalties thereon or therefor.

This application is a division of application Ser. No. 809,599 filed 24June 1977 now U.S. Pat. No. 4,107,406 for Improved Aluminum Alloy forPrimary Alkaline Fuel Cells and Batteries.

BACKGROUND OF THE INVENTION

The present invention generally relates to anodes in electrochemicalcells using strong alkaline solutions and/or seawater as electrolytesand more particularly to an aluminum alloy anode having high energy,high electrochemical potential and low gassing rates in these cells.

The majority of previous aluminum alloys contain, most importantly,fractional percentages of tin, gallium, mercury and magnesium. Graphicalanalysis of recent data led to the inventors predicting significantimprovement to the desired properties of high potential and low gassingby producing an aluminum alloyed with only gallium for high potentialand low gassing, and magnesium to control tensile modulus. The exclusionof tin as an alloying ingredient appears to raise the potential andlower the gassing rate as compared to similar alloys containing tin inthe range of 0.05 to 0.34 percent.

Another prior alloy contains alluminum, mercury and various otherelements. Mercury, although well known for its ability to raise thehydrogen overvoltage of other metals, is quite toxic under certainconditions. The U.S. Navy is spending large sums of money to eliminatethe use of mercury in many batteries.

SUMMARY OF THE INVENTION

It is therefore a general object of the present invention to provide animproved anode suitable for electrolytic cells. It is an additionalobject to provide an improved anode material for a silver oxide-aluminumbattery. Further objects are that the anode be suitable for highelectrochemical potential and have a low gassing rate. These and otherobjects of the invention and the various features and details ofconstruction and operation will become apparent from the specification.

The above objects are obtained by providing an aluminum base alloy withspecific amounts of gallium and magnesium. The addition of gallium at apreferred level provides an anode suitable for producing a highelectrochemical potential with reduced gassing. The addition ofmagnesium at a preferred level increases the tensile modulus whensuitably heat treated. When the alloy is rolled and annealed, it isuniquely suited by virtue of its high potential and low gassing rate asan anode in primary alkaline batteries and fuel cells.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE depicts the changes in volts/cell and gas generation in CC H₂/CM² /MIN as a function of changes in tin and gallium content.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, there is provided a FIGUREthat shows an anode that is alloyed from aluminum having an initial highpurity of 99.997 percent. Magnesium varying from 0.5 to 1.2 percent asan additive is also included in the anode. The FIGURE is a graph drawnafter measuring a plurality of samples showing the effect of gallium andtin content as additional alloying ingredients used in the anode.

Curve 10 of the FIGURE shows the amounts of tin and gallium contentresulting in corrosion producing gas at the rate of 0.1 CC H₂ /CM² /MIN.Curve 12 shows reduced gas generation of only 0.05 CC H₂ /CM² /MIN.Curves 14, 16 and 18 show cell potentials of 1.70, 1.65 and 1.60volts/cell under load. All measurements were taken using the test setupexplained in Example 1.

EXAMPLE 1

In all cell tests for forming the FIGURE the same test setup was used.The current output of the battery was measured across a low impedanceshunt resistor. The current density was 3 amps/in² (0.47 amps CM²). Thefull cell voltahe variance was measured. The anode and cathod half cellvoltages were measured to characterize the two electrodes. Thetemperature of the electrolyte was held constant throughout the tests at150° F. (65.6° C.). All tests were conducted with a 25 percent KOHsolution prepared with synthetic seawater made to standard with ASTM seasalt to which 20 gm/liter of sodium stannate was added. The alloycompositions of both gallium and tin were varied in the anode. Theremaining elements consisted of super high purity aluminum having aninitial purity of 99.997 percent and a varying amount of magnesium. Themagnesium was varied from 0.5 to 1.2 percent. This is not shown in thegraph as no detectable changes in electrochemical measurement werenoticed over this range. The only value of the magnesium is for strengthcharacteristcs.

EXAMPLE 2

Using the test setup of Example 1, an alloy consisting essentially ofaluminum that was initially 99.997 percent pure, 0.04 percent galliumand 0.74 percent magnesium was used. A cell voltage of 1.71 volts/cellwas obtained. This concurs with the expected results of the FIGURE.

It is shown in the FIGURE that as the amount of tin is reduced to zero,less gallium content is necessary to maintain an equivalent voltage percell. For example, to maintain 1.65 volts/cell with 0.14 percent in, aquantity of 0.03 percent gallium is necessary. If the alloy coantains notin, then only approximately 0.01 percent gallium content is necessary.The results obtained at 1.60 volts/cell and 1.70 volts/cell also showthe negative effect on performance produced by tin.

Another adverse effect of tin is shown in the FIGURE on the gassing rateof anodes where 0.55 CC H₂ /CM² /MIN is produced with a 0.15 percent tinand 0.06 percent gallium at 1.70 volts/cell. If the amount of tin isreduced or eliminated, less gallium is required to maintain 1.70volts/cell and the H₂ gassing rate is reduced. Slightly increasing thetin content above the 0.15 percent level at all concentrations ofgallium results in an unacceptable level of gassing.

There has, therefore, been described appropriate alloys for the anodesin high power density, potassium hydroxide enhanced, seawater cells. Onecan also notice, with the aid of the FIGURE, inappropriate combinationsof alloys in which poor performance is obtained. The FIGURE aids ingiving a far better understanding of performance than has heretoforebeen available using isolated examples to explain results obtained fromvarious combinations of materials. The FIGURE shows adequate performancewith a tin content from 0 to 0.19 percent by weight and a galliumcontent of 0.001 to 0.072 percent by weight. However, improvedperformance was obtained when tin was eliminated. A range of superiorperformance was attained when the gallium content was between 0.02 and0.06 percent by weight and no tin was used.

What is claimed is:
 1. An anode for a primary electrochemical energysource consisting essentially of 0.001 to 0.072 percent by weight ofgallium and the balance of aluminum having an initial purity of at least99.997 percent.
 2. An anode for a primary electrochemical energy sourceconsisting essentially of 0.001 to 0.072 percent by weight of galliumand the balance of aluminum having incidental impurities.
 3. An anodefor a primary electrochemical energy source consisting essentially of0.02 to 0.06 percent by weight of gallium and the balance of aluminumhaving an initial purity of at least 99.997 percent.
 4. An anode for aprimary electrochemical energy source consisting essentially of 0.02 to0.06 percent by weight of gallium and the balance of aluminum havingincidental impurities.